<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
        }

        .search {
            position: relative;
            width: 178px;
            margin: 100px;
        }

        .con {
            /* 先隐藏 */
            display: none;
            position: absolute;
            top: -40px;
            width: 171px;
            border: 1px solid rgba(0, 0, 0, .2);
            box-shadow: 0 2px 4px rgba(0, 0, 0, .2);
            padding: 5px 0;
            font-size: 18px;
            line-height: 20px;
            color: #333;
        }

        .con::before {
            content: '';
            width: 0;
            height: 0;
            position: absolute;
            top: 0px;
            left: 18px;
            border: 8px solid #000;
            border-style: solid dashed dashed;
            border-color: #fff transparent transparent;
        }

        input {
            margin-top: 100px;
        }
    </style>
</head>

<body>
    <div class="search">
        <div class="con" style="width: 800px; height: 100px; font-size: 80px; background-color: skyblue;">默认内容</div>
        <input type="text" placeholder="请输入您的快递单号" class="jd"
            style="width: 800px; height: 60px; top:200px; font-size: 50px;">
    </div>
    <script>
        /* 
            需求:
                 1. input 获取焦点，如果有内容，把 input 的值放入 div 并显示。
                    如果 input 的值为空，则div隐藏
                 2. input 失去焦点，div 隐藏
                 3. 表单输入文字，则div显示

            注意: keydown、keypress 事件触发，文字还没有落入文本框。    
        */
        let input = document.querySelector('input');
        let div = document.querySelector('.con');

        // 表单获取焦点
        input.addEventListener('focus', function () {
            let value = this.value;
            if (value == '') {
                div.style.display = 'none';
            } else {
                div.style.display = 'block';
            }
        });
        // 输入文字
        input.addEventListener('keyup', function () {
            let value = this.value;
            if (value != '') {
                div.innerHTML = value;
                div.style.display = 'block';
            } else {
                div.style.display = 'none';
            }
        });

        // 表单失去焦点
        input.addEventListener('blur', function () {
            div.style.display = 'none';
        });

    </script>
</body>